The production of integrated circuits begins with the creation of high-quality semiconductor wafers. During the wafer fabrication process, the wafers may undergo multiple masking, etching, and dielectric and conductor deposition processes. Because of the high-precision required in the production of these integrated circuits, an extremely flat surface is generally needed on at least one side of the semiconductor wafer to ensure proper accuracy and performance of the microelectronic structures being created on the wafer surface. As the size of integrated circuits continues to decrease and the number of microstructures per integrated circuit increases, the need for precise wafer surfaces becomes more important. Therefore, between each processing step, it is usually necessary to polish or planarize the surface of the wafer to obtain the flattest surface possible.
For a discussion of chemical mechanical planarization (CMP) processes and apparatus, see, for example, Arai, et al., U.S. Pat. No. 4,805,348, issued February, 1989; Arai, et al., U.S. Pat. No. 5,099,614, issued March, 1992; Karlsrud, et al., U.S. Pat. No. 5,329,732, issued July, 1994; Kailsrud et al., U.S. Pat. No. 5,498,196, issued March, 1996; and Karlsrud, et al, U.S. Pat. No. 5,498,199, issued March, 1996.
Such polishing is well known in the art and generally includes attaching one side of the wafer to a flat surface of a wafer carrier or chuck and pressing the other side of the wafer against a flat polishing surface. In general, the polishing surface includes a polishing pad that has an exposed abrasive surface of, for example, cerium oxide, aluminum oxide, filmed/precipitated silica or other particulate abrasives. Commercially available polishing pads can be formed of various materials known in the art. Typically, a polishing pad may be formed from a blown polyurethane, such as the IC and GS series of polishing pads available from Rodel Products Corporation in Scottsdale, Ariz. The hardness, density, color, reflectivity, and other characteristics of the polishing pad may vary from application to application, e.g., according to the material that is to be polished.
During the polishing or planarization process, the workpiece or wafer is typically pressed against the polishing pad surface while the pad rotates about its vertical axis. In addition, to improve the polishing effectiveness, the wafer may also be rotated about its vertical axis and oscillated back and forth over the surface of the polishing pad. It is well known that polishing pads tend to wear unevenly during the polishing operation, causing surface irregularities to develop on the pad. To ensure consistent and accurate planarization and polishing of all workpieces, these irregularities must be removed.
During the CMP process, workpieces occasionally become dislodged from the workpiece carrier, or they may break during polishing. If a dislodged workpiece, a part of a broken workpiece, or other extraneous material is allowed to remain on the polishing table, it could contact other workpieces and/or workpiece carriers on the same polishing table and thereby damage or destroy all of the workpieces on the table. Accordingly, it is desirable to detect the presence of a broken or dislodged workpiece immediately and to terminate processing until the situation can be rectified. Typically, this requires a thorough cleaning and/or replacement of the polishing pad, so that workpiece fragments and other debris can be removed so that they do not damage other intact workpieces.
Presently known systems for detecting the loss of workpieces or for detecting broken workpieces are unsatisfactory in several regards. For example, currently known systems may be limited to operation with a small number of similarly colored polishing pads. Such known systems may be ineffective for detecting wafer loss on a dark colored polishing pad or in an environment where the polishing pad may become discolored over time. Present systems may also be inadequate in CMP environments that employ a large amount of polishing slurry and/or polishing slurry having a variety of colors. Furthermore, the presence of slurry, deionized water, iodine (or other CMP compounds) on the pad and on the wafer itself tend to mask the reflected light signal, preventing the signal from being properly detected by the photo detector. Consequently, many presently known workpiece detection schemes often emit "false" readings whereupon machines are shut down and processing halted even though all disks remain intact within their respective carriers.
Therefore, a technique for detecting lost or dislodged workpieces on a CMP polishing pad is thus needed which overcomes the shortcomings of the prior art.